Exergy analysis of underground coal gasification with simultaneous storage of carbon dioxide

Various options are considered to reduce CO₂ emissions when utilizing deep coal by applying underground coal gasification (UCG), i.e., in combination with carbonation of synthetic minerals (CaO), conventional UCG followed by ex-situ separation of CO₂ and upgrading the product gas using naturally occurring minerals (wollastonite). A chemical equilibrium model was used to analyze the effect of process parameters on product composition and use it for an exergy analysis. The result is presented in terms of theoretical (ideal unit operations), practical (state of the art technology), and zero-emission (applying current CO₂ capture and sequestration technology (CCS) to all sources of CO₂ emission) recovery factors. The results show that underground gasification of deep coal can optimally extract 52–68% of the coal chemical exergy, but zero-emission extraction gives a negative recovery indicating that it is not practical with current state of the art CCS technology. Using in-situ CaO, which will enhance the H₂ production, is theoretically feasible with a recovery factor around 80%, but is not exergetically feasible with the current state of technology. Ex-situ upgrading of the conventional UCG product gas with wollastonite is exergetically feasible for both practical and zero-emission cases according to the equilibrium model.